1. Field of the Invention
The present invention is related to antenna design. More particularly, the present invention is related to antenna structures that are capable of increased operational bandwidth without a corresponding increase in antenna size.
2. Background of the Invention
In the design of an antenna, it is well known that the size of the antenna and the bandwidth over which it can operate are in competition. As the antenna dimensions are reduced below a half wavelength, bandwidth decreases quite rapidly. However, for applications like portable wireless communications and covert military scenarios, it is desirable to keep the antenna as compact as possible while meeting increasingly large bandwidth requirements for modern digital communications.
More specifically, wireless and mobile-wireless communications are a critical and expanding technology in both military and commercial markets. Efficient, compact antennas are crucial elements of these systems. Antenna arrays can be beneficial for base stations and vehicular applications, but for operations below 2 GHz, the required size for an array makes them unsuitable for handheld units and items like artillery-delivered, unattended ground sensors (UGS). Small antennas are also less conspicuous, a plus for UGS hoping for concealment as well as for commercial applications where cosmetic appearances are important.
In addition to size, the frequency bandwidth supported by these antennas is very important. Of course, the antenna must support the information bandwidth, which determines how much and how fast data can be exchanged. But it must also support the signal bandwidth, which may be increased by spread spectrum requirements. Moreover, radios such as the military's Joint Tactical Radio (JTR) are expected to actively alter their operating frequencies in response to the presence of other transmissions, further extending the frequencies over which the antenna must operate. The JTR is also expected to support simultaneous operation in several modes (voice, data, video), further increasing the demand for bandwidth. Finally, new technologies like ultra-wideband techniques for communications and sensing applications depend critically on receiving and transmitting broadband signals. There is a clear need for antennas that can handle broad ranges of the frequency spectrum.
Unfortunately, as mentioned above, broad bandwidth and small size are conflicting requirements for an antenna. Widely recognized performance bounds relating bandwidth and antenna size are well-known. Specifically, there is a bound on the amount of bandwidth that can be achieved as a function of antenna size. While quite helpful, available studies are silent on the question of how to construct an antenna element capable of operating near the performance bound.
The most successful attempts to date to construct small antennas with the widest possible bandwidths have involved variations on a monopole antenna that is top loaded with a disk like that illustrated in FIG. 1. Variants of this type of antenna have been investigated and reported in G. Goubau, “Multi-element monopole antennas,” Proc. ECOM-ARO Workshop on Electrically Small Antennas, Fort Monmouth, N.J., May 6 and 7, 1976, G. Goubau and F. K. Schwering, Eds., pp. 63-67; C. H. Friedman, “Wide-Band Matching of a Small Disk-Loaded Monopole,” IEEE Trans. Antennas and Propagat., vol. AP-33, no. 10, October 1985; and H. D. Foltz, J. S. McLean and G. Crook, “Disk-Loaded Monopoles with Parallel Strip Elements,” IEEE Trans. Antennas and Propagat., vol. 46, no. 12, December 1998.
Foltz recently reported a scheme in which monopoles that were on the order of λ/15-tall yielded full-width half-power (FWHP) bandwidths of as much as 41% as compared to a theoretical upper bound of 50%. The −10 dB bandwidths were a more modest 10%. Foltz reported a second antenna designed for a different frequency band with, a 24% FWHP bandwidth as compared to a 34% theoretical upper bound and a 12% −10 dB bandwidth. Even these best results are 20% to 30% below the FWHM theoretical bound, leaving appreciable room for further improvement.
In his investigation, Foltz emphasized small size over bandwidth. For a doubling in the size of the antenna, the theory calls for a 5× improvement in bandwidth. Hence, it is believed that octave bandwidths with antennas shorter than λ/7 can be achieved. At 2 GHz, such an antenna would be less than an inch tall.
Foltz's solution is also commendable for realizing a second-order matching network within the antenna structure itself, as opposed to requiring additional tuning elements.
Despite the advances made in this field of antenna design, there remains a desire to further improve the performance of disk loaded, monopole antennas.